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                <h1 id="Java泛型"><a href="#Java泛型" class="headerlink" title="Java泛型"></a>Java泛型</h1><p>面试题：泛型的考点，问题我不太记得了，还是想 &lt; Integer&gt; 和 &lt; String&gt; 的对象是否相等</p>
<blockquote>
<p><a target="_blank" rel="noopener" href="https://snailclimb.gitee.io/javaguide/#/docs/java/basis/Java%E5%9F%BA%E7%A1%80%E7%9F%A5%E8%AF%86?id=java-%E6%B3%9B%E5%9E%8B%E4%BA%86%E8%A7%A3%E4%B9%88%EF%BC%9F%E4%BB%80%E4%B9%88%E6%98%AF%E7%B1%BB%E5%9E%8B%E6%93%A6%E9%99%A4%EF%BC%9F%E4%BB%8B%E7%BB%8D%E4%B8%80%E4%B8%8B%E5%B8%B8%E7%94%A8%E7%9A%84%E9%80%9A%E9%85%8D%E7%AC%A6%EF%BC%9F">Java 泛型了解么？什么是类型擦除？介绍一下常用的通配符？ (gitee.io)</a></p>
<p><a target="_blank" rel="noopener" href="https://www.cnblogs.com/coprince/p/8603492.html">java 泛型详解-绝对是对泛型方法讲解最详细的，没有之一 - little fat - 博客园 (cnblogs.com)</a></p>
<p><a target="_blank" rel="noopener" href="https://blog.csdn.net/sunxianghuang/article/details/51982979">Java泛型深入理解_void-CSDN博客_java泛型</a></p>
<p><a target="_blank" rel="noopener" href="https://www.zhihu.com/question/20400700">Java 泛型  中 super 怎么 理解？与 extends 有何不同？ - 知乎 (zhihu.com)</a></p>
</blockquote>
<p>需要了解的：</p>
<ul>
<li>Java泛型作用，使用</li>
<li>类型擦除</li>
<li>通配符，父类子类上下边界</li>
</ul>
<h2 id="简介"><a href="#简介" class="headerlink" title="简介"></a>简介</h2><p>本质：<strong>参数化类型</strong>，也就是将数据类型变成一个<strong>参数</strong></p>
<p>Java的泛型是<strong>伪泛型</strong>，因为在编译期间所有的泛型信息都会被擦掉，也就是<strong>类型擦除</strong>。</p>
<h2 id="类型擦除"><a href="#类型擦除" class="headerlink" title="类型擦除"></a>类型擦除</h2><p><strong>泛型只在编译阶段有效</strong>。看下面的代码：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">></span></span> stringArrayList <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Integer</span><span class="token punctuation">></span></span> integerArrayList <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Integer</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token class-name">Class</span> classStringArrayList <span class="token operator">=</span> stringArrayList<span class="token punctuation">.</span><span class="token function">getClass</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Class</span> classIntegerArrayList <span class="token operator">=</span> integerArrayList<span class="token punctuation">.</span><span class="token function">getClass</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token keyword">if</span><span class="token punctuation">(</span>classStringArrayList<span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span>classIntegerArrayList<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
    <span class="token class-name">Log</span><span class="token punctuation">.</span><span class="token function">d</span><span class="token punctuation">(</span><span class="token string">"泛型测试"</span><span class="token punctuation">,</span><span class="token string">"类型相同"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>输出结果：<code>D/泛型测试: 类型相同</code>。</p>
<p>通过上面的例子可以证明，在编译之后程序会采取去泛型化的措施。也就是说Java中的泛型，只在编译阶段有效。<strong>在编译过程中，正确检验泛型结果后，会将泛型的相关信息擦出，并且在对象进入和离开方法的边界处添加==类型检查和类型转换的方法==。</strong>也就是说，泛型信息不会进入到运行时阶段。</p>
<p>对此总结成一句话：<strong>泛型类型在逻辑上看以看成是多个不同的类型，实际上都是相同的基本类型</strong>。</p>
<h2 id="使用"><a href="#使用" class="headerlink" title="使用"></a>使用</h2><p>一般有三种使用方式：泛型类，泛型接口，泛型方法。</p>
<p>泛型的类型参数只能是类类型，不能是简单类型。</p>
<h3 id="1-泛型类"><a href="#1-泛型类" class="headerlink" title="1. 泛型类"></a>1. 泛型类</h3><p>类声明</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token comment">// T可以使用任意标识，见下面</span>
<span class="token comment">// 实例化泛型类的时候，必须指定T具体类型</span>
<span class="token comment">// 类名&lt;泛型></span>
<span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Generic</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">></span></span><span class="token punctuation">&#123;</span>
    <span class="token keyword">private</span> <span class="token class-name">T</span> key<span class="token punctuation">;</span>
    
    <span class="token comment">// 构造，set，get等</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>类实例化：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token class-name">Generic</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Integer</span><span class="token punctuation">></span></span> g <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Generic</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Integer</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<h3 id="2-泛型接口"><a href="#2-泛型接口" class="headerlink" title="2. 泛型接口"></a>2. 泛型接口</h3><p>其实和泛型类差不多啦。</p>
<p>接口声明：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">interface</span> <span class="token class-name">Generator</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">></span></span><span class="token punctuation">&#123;</span>
    <span class="token keyword">public</span> <span class="token class-name">T</span> <span class="token function">method</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span></span></code></pre>

<p>接口实现，可以<strong>不指定类型</strong>：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">class</span> <span class="token class-name">GeneratorImpl</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">></span></span> <span class="token keyword">implements</span> <span class="token class-name">Generator</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">></span></span><span class="token punctuation">&#123;</span>
    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">public</span> <span class="token class-name">T</span> <span class="token function">method</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
        <span class="token keyword">return</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>接口实现，<strong>指定类型</strong>：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">class</span> <span class="token class-name">GeneratorImpl</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">></span></span> <span class="token keyword">implements</span> <span class="token class-name">Generator</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">></span></span><span class="token punctuation">&#123;</span>
    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">public</span> <span class="token class-name">String</span> <span class="token function">method</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
        <span class="token keyword">return</span> <span class="token string">"hello"</span><span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>注意：这里实现类中的 T，与接口中的 T 不是同一个了，原来接口中的T在这都变成 <code>String</code> 了。</p>
<h3 id="3-泛型方法"><a href="#3-泛型方法" class="headerlink" title="3. 泛型方法"></a>3. 泛型方法</h3><p>方法声明：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">E</span><span class="token punctuation">></span></span> <span class="token keyword">void</span> <span class="token function">printArray</span><span class="token punctuation">(</span><span class="token class-name">E</span><span class="token punctuation">[</span><span class="token punctuation">]</span> inputArray<span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
    <span class="token keyword">for</span><span class="token punctuation">(</span><span class="token class-name">E</span> e <span class="token operator">:</span> inputArray<span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
        <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">printf</span><span class="token punctuation">(</span><span class="token string">"%s"</span><span class="token punctuation">,</span>e<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>方法使用：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token comment">// 创建不同类型数组： Integer, Double 和 Character</span>
<span class="token class-name">Integer</span><span class="token punctuation">[</span><span class="token punctuation">]</span> intArray <span class="token operator">=</span> <span class="token punctuation">&#123;</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span> <span class="token punctuation">&#125;</span><span class="token punctuation">;</span>
<span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> stringArray <span class="token operator">=</span> <span class="token punctuation">&#123;</span> <span class="token string">"Hello"</span><span class="token punctuation">,</span> <span class="token string">"World"</span> <span class="token punctuation">&#125;</span><span class="token punctuation">;</span>
<span class="token function">printArray</span><span class="token punctuation">(</span> intArray  <span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token function">printArray</span><span class="token punctuation">(</span> stringArray  <span class="token punctuation">)</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>需要注意的几点：</p>
<ul>
<li>只有在返回类型之前声明了 <code>&lt;T&gt;</code> 的才是泛型方法</li>
<li>静态方法无法访问类上定义的泛型，如果静态方法要使用泛型的话，必须将静态方法也定义成泛型方法 </li>
</ul>
<p>举例：</p>
<p>只有在返回类型之前声明了 <code>&lt;T&gt;</code> 的才是泛型方法！其余的都不是。</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Generic</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">></span></span><span class="token punctuation">&#123;</span>     
        <span class="token keyword">private</span> <span class="token class-name">T</span> key<span class="token punctuation">;</span>

        <span class="token keyword">public</span> <span class="token class-name">Generic</span><span class="token punctuation">(</span><span class="token class-name">T</span> key<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
            <span class="token keyword">this</span><span class="token punctuation">.</span>key <span class="token operator">=</span> key<span class="token punctuation">;</span>
        <span class="token punctuation">&#125;</span>

        <span class="token comment">//我想说的其实是这个，虽然在方法中使用了泛型，但是这并不是一个泛型方法。</span>
        <span class="token comment">//这只是类中一个普通的成员方法，只不过他的返回值是在声明泛型类已经声明过的泛型。</span>
        <span class="token comment">//所以在这个方法中才可以继续使用 T 这个泛型。</span>
        <span class="token keyword">public</span> <span class="token class-name">T</span> <span class="token function">getKey</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
            <span class="token keyword">return</span> key<span class="token punctuation">;</span>
        <span class="token punctuation">&#125;</span>
        <span class="token comment">//这也不是一个泛型方法，这就是一个普通的方法，只是使用了Generic&lt;Number>这个泛型类做形参而已。</span>
    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">showKeyValue1</span><span class="token punctuation">(</span><span class="token class-name">Generic</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Number</span><span class="token punctuation">></span></span> obj<span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
        <span class="token class-name">Log</span><span class="token punctuation">.</span><span class="token function">d</span><span class="token punctuation">(</span><span class="token string">"泛型测试"</span><span class="token punctuation">,</span><span class="token string">"key value is "</span> <span class="token operator">+</span> obj<span class="token punctuation">.</span><span class="token function">getKey</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span>
    
     <span class="token comment">/** 
     * 这才是一个真正的泛型方法。
     * 首先在public与返回值之间的&lt;T>必不可少，这表明这是一个泛型方法，并且声明了一个泛型T
     * 这个T可以出现在这个泛型方法的任意位置.
     * 泛型的数量也可以为任意多个 
     *    如：public &lt;T,K> K showKeyName(Generic&lt;T> container)&#123;
     *        ...
     *        &#125;
     */</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<h2 id="常见通配符"><a href="#常见通配符" class="headerlink" title="常见通配符"></a>常见通配符</h2><ul>
<li>？ 不确定的Java类型</li>
<li>T (type) 具体的一个Java类型</li>
<li>K V (key,value) 代表Java键值中的 Key 与 Value</li>
<li>E (element) 代表 Element</li>
</ul>
<h3 id="泛型限定通配符-上下边界"><a href="#泛型限定通配符-上下边界" class="headerlink" title="泛型限定通配符/上下边界"></a>泛型限定通配符/上下边界</h3><ul>
<li>限定通配符：<ul>
<li>上边界：<code>&lt;? extends Number&gt;</code> ，只能是 <code>Number</code> 的子类<br><img src="https://gitee.com/songx86/SongPicBed/raw/master/img/image-20210531113912914.png" alt="上边界extends"></li>
<li>下边界：<code>&lt;? super Integer&gt;</code>，只能是 <code>Integer</code> 的父类<br><img src="https://gitee.com/songx86/SongPicBed/raw/master/img/image-20210531113836697.png" alt="下边界super"></li>
</ul>
</li>
<li>非限定通配符：<code>&lt;?&gt;</code> 代表所有类型</li>
</ul>
<p>举例：</p>
<p>为泛型添加上边界，即传入的类型实参必须是指定类型的子类型：</p>
<p>声明：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">showKeyValue1</span><span class="token punctuation">(</span><span class="token class-name">Generic</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span> <span class="token keyword">extends</span> <span class="token class-name">Number</span><span class="token punctuation">></span></span> obj<span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
    <span class="token class-name">Log</span><span class="token punctuation">.</span><span class="token function">d</span><span class="token punctuation">(</span><span class="token string">"泛型测试"</span><span class="token punctuation">,</span><span class="token string">"key value is "</span> <span class="token operator">+</span> obj<span class="token punctuation">.</span><span class="token function">getKey</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span></span></code></pre>

<p>使用：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token class-name">Generic</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">></span></span> generic1 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Generic</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token string">"11111"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Generic</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Integer</span><span class="token punctuation">></span></span> generic2 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Generic</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Integer</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token number">2222</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Generic</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Float</span><span class="token punctuation">></span></span> generic3 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Generic</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Float</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token number">2.4f</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Generic</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Double</span><span class="token punctuation">></span></span> generic4 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Generic</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Double</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token number">2.56</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">//这一行代码编译器会提示错误，因为String类型并不是Number类型的子类</span>
<span class="token comment">//showKeyValue1(generic1);</span>

<span class="token function">showKeyValue1</span><span class="token punctuation">(</span>generic2<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token function">showKeyValue1</span><span class="token punctuation">(</span>generic3<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token function">showKeyValue1</span><span class="token punctuation">(</span>generic4<span class="token punctuation">)</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>上下边界要添加在声明上，而不是参数上：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token comment">//在泛型方法中添加上下边界限制的时候，必须在权限声明与返回值之间的&lt;T>上添加上下边界，即在泛型声明的时候添加</span>
<span class="token comment">//public &lt;T> T showKeyName(Generic&lt;T extends Number> container)，编译器会报错："Unexpected bound"</span>
<span class="token keyword">public</span> <span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span> <span class="token keyword">extends</span> <span class="token class-name">Number</span><span class="token punctuation">></span></span> <span class="token class-name">T</span> <span class="token function">showKeyName</span><span class="token punctuation">(</span><span class="token class-name">Generic</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">></span></span> container<span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
    <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"container key :"</span> <span class="token operator">+</span> container<span class="token punctuation">.</span><span class="token function">getKey</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token class-name">T</span> test <span class="token operator">=</span> container<span class="token punctuation">.</span><span class="token function">getKey</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">return</span> test<span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<h3 id="泛型通配符-？"><a href="#泛型通配符-？" class="headerlink" title="泛型通配符 ？"></a>泛型通配符 <code>？</code></h3><p><code>Integer</code> 是 <code>Number</code> 的一个子类</p>
<p>泛型类 <code>Generic&lt;Integer&gt;</code> 是 <code>Generic&lt;Number&gt;</code> 的子类吗？能用 <code>Generic&lt;Integer&gt;</code> 传入 <code>Generic&lt;Number&gt;</code> 吗？</p>
<p>答案是不行的。</p>
<p>首先声明一个泛型方法：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">showKeyValue1</span><span class="token punctuation">(</span><span class="token class-name">Generic</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Number</span><span class="token punctuation">></span></span> obj<span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
    <span class="token class-name">Log</span><span class="token punctuation">.</span><span class="token function">d</span><span class="token punctuation">(</span><span class="token string">"泛型测试"</span><span class="token punctuation">,</span><span class="token string">"key value is "</span> <span class="token operator">+</span> obj<span class="token punctuation">.</span><span class="token function">getKey</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span></span></code></pre>

<p> 然后进行 Integer 与 Number 的测试：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token class-name">Generic</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Integer</span><span class="token punctuation">></span></span> gInteger <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Generic</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Integer</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token number">123</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Generic</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Number</span><span class="token punctuation">></span></span> gNumber <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Generic</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Number</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token number">456</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token function">showKeyValue</span><span class="token punctuation">(</span>gNumber<span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">// showKeyValue(gInteger);</span>

<span class="token comment">// showKeyValue(gInteger)这里编译器会为我们报错：Generic&lt;java.lang.Integer> cannot be applied to Generic&lt;java.lang.Number></span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>那么要怎么才能让 <code>showKeyValue(gNumber)</code> 和 <code>showKeyValue(gInteger)</code> 都不报错呢？</p>
<p>答案是使用通配符 <code>?</code> ，将参数改为 <code>Generic&lt;?&gt; obj</code> 即可：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">showKeyValue1</span><span class="token punctuation">(</span><span class="token class-name">Generic</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span><span class="token punctuation">></span></span> obj<span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
    <span class="token class-name">Log</span><span class="token punctuation">.</span><span class="token function">d</span><span class="token punctuation">(</span><span class="token string">"泛型测试"</span><span class="token punctuation">,</span><span class="token string">"key value is "</span> <span class="token operator">+</span> obj<span class="token punctuation">.</span><span class="token function">getKey</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span></span></code></pre>

<p>类型通配符一般是使用 <code>?</code> 代替具体的<strong>类型实参</strong>。</p>
<p>注意了，此处 <code>?</code> 是类型实参，而不是类型形参。</p>
<p>再直白点的意思就是，此处的 <code>？</code> 和Number、String、Integer一样<strong>都是一种实际的类型</strong>，可以把 <code>？</code> 看成所有类型的父类，是一种真实的类型。</p>
<h2 id="常见面试题"><a href="#常见面试题" class="headerlink" title="常见面试题"></a>常见面试题</h2><ol>
<li><p>Java中的泛型是什么 ? 使用泛型的好处是什么?<br>泛型是一种参数化类型的机制。它可以使得<strong>代码适用于各种类型</strong>，从而编写更加通用的代码，例如集合框架。</p>
<p>泛型是一种<strong>编译时类型确认</strong>机制。它提供了<strong>编译期的类型安全</strong>，确保在泛型类型（通常为泛型集合）上只能使用正确类型的对象，<strong>避免了在运行时出现ClassCastException</strong>。</p>
</li>
</ol>
<p>2、Java的泛型是如何工作的 ? 什么是类型擦除 ?</p>
<p>​    泛型的正常工作是依赖编译器在<strong>编译源码的时候</strong>，先进行<strong>类型检查</strong>，然后进行<strong>类型擦除</strong>并且在类型参数出现的地方<strong>插入强制转换</strong>的相关指令实现的。</p>
<p>​    编译器在编译时擦除了所有类型相关的信息，所以在运行时不存在任何类型相关的信息。<strong>例如 <code>List&lt;String&gt;</code> 在运行时仅用一个List类型来表示。</strong>为什么要进行擦除呢？这是为了避免类型膨胀。</p>
<ol start="3">
<li><p>什么是泛型中的限定通配符和非限定通配符 ?<br>限定通配符对类型进行了限制。</p>
<ul>
<li>两种<strong>限定通配符</strong><ul>
<li><code>&lt;? extends T&gt;</code>它通过确保类型必须是T的子类来设定类型的<strong>上界</strong></li>
<li><code>&lt;? super T&gt;</code>它通过确保类型必须是T的父类来设定类型的<strong>下界</strong>。<br>泛型类型必须用限定内的类型来进行初始化，否则会导致编译错误。</li>
</ul>
</li>
<li>另一方面<code>&lt;?&gt;</code>表示了<strong>非限定通配符</strong>，因为<code>&lt;?&gt;</code>可以用<strong>任意类型</strong>来替代。</li>
</ul>
</li>
<li><p>List&lt;? extends T&gt;和List &lt;? super T&gt;之间有什么区别 ?</p>
</li>
<li><p>如何编写一个泛型方法，让它能接受泛型参数并返回泛型类型?</p>
</li>
<li><p>Java中如何使用泛型编写带有参数的类?</p>
</li>
<li><p>编写一段泛型程序来实现LRU缓存?</p>
</li>
<li><p>你可以把 <code>List&lt;String&gt;</code> 传递给一个接受 <code>List&lt;Object&gt;</code> 参数的方法吗？</p>
</li>
<li><p>Array中可以用泛型吗?<br><strong>Array不支持泛型，而List支持</strong>。List可以提供编译期的类型安全保证，而Array却不能。</p>
</li>
<li><p>如何阻止Java中的类型未检查的警告?<br>如果你把<strong>泛型和原始类型混合起来使用</strong>，例如下列代码，Java 5的javac编译器会产生类型未检查的警告<br>，例如 <code> List&lt;String&gt; rawList = new ArrayList()</code><br>注意: Hello.java使用了<strong>未检查或称为不安全的操作</strong>;<br>这种警告可以使用 <code>@SuppressWarnings(&quot;unchecked&quot;)</code> <strong>注解</strong>来屏蔽。</p>
</li>
<li><p>Java中 <code>List&lt;Object&gt;</code> 和原始类型 List 之间的区别?</p>
<ol>
<li>原始类型和带参数类型<code>&lt;Object&gt;</code>之间的主要区别是，在编译时编译器<strong>不会对原始类型进行类型安全检查，却会对带参数的类型进行检查</strong>，通过使用Object作为类型，可以告知编译器该方法可以接受任何类型的对象，比如String或Integer。这道题的考察点在于对泛型中原始类型的正确理解。</li>
<li>它们之间的第二点区别是，你可以把<strong>任何带参数的泛型类型传递给接受原始类型List的方法</strong>，但却不能把<code>List&lt;String&gt;</code>传递给接受<code>List&lt;Object&gt;</code>的方法，因为会产生编译错误。</li>
</ol>
</li>
<li><p><code>Java中List&lt;?&gt;</code>和 <code>List&lt;Object&gt;</code> 之间的区别是什么?<br>这道题跟上一道题看起来很像，实质上却完全不同。<br><code>List&lt;?&gt; </code>是一个<strong>所有类型的List</strong>，而<code>List&lt;Object&gt;</code>其实是<strong>Object类型的List</strong>。<br>你可以把<code>List&lt;String&gt;, List&lt;Integer&gt;</code>赋值给<code>List&lt;?&gt;</code>，却不能把<code>List&lt;String&gt;</code>赋值给<code>List&lt;Object&gt;</code>。   </p>
</li>
</ol>
<pre><code>`List&lt;Object&gt;` 与 `List&lt;?&gt;` 并不等同，`List&lt;Object&gt;` 是`List&lt;?&gt;`的子类。还有不能往`List&lt;?&gt; list`里添加任意对象，除了null。
</code></pre>
<ol start="13">
<li><p><code>List&lt;String&gt;</code>和原始类型List之间的区别.<br>该题类似于“11. 原始类型和带参数类型之间有什么区别”。<br><strong>带参数类型是类型安全</strong>的，而且其类型安全是由<strong>编译器保证</strong>的，但<strong>原始类型List却不是类型安全的</strong>。<br>你不能把String之外的任何其它类型的Object存入String类型的List中，而你可以把任何类型的对象存入原始List中。<br>使用泛型的带参数类型你<strong>不需要进行类型转换</strong>，但是对于原始类型，你则需要进行<strong>显式的类型转换</strong>。</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token class-name">List</span> listOfRawTypes <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
listOfRawTypes<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">"abc"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
listOfRawTypes<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token number">123</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">//编译器允许这样 - 运行时却会出现异常</span>
<span class="token class-name">String</span> item <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">)</span> listOfRawTypes<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">//需要显式的类型转换</span>
item <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">)</span> listOfRawTypes<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">//抛ClassCastException，因为Integer不能被转换为String</span>
      
<span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">></span></span> listOfString <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
listOfString<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">"abcd"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
listOfString<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token number">1234</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">//编译错误，比在运行时抛异常要好</span>
item <span class="token operator">=</span> listOfString<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">//不需要显式的类型转换 - 编译器自动转换</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre></li>
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                            Hexo部署至阿里云服务器的，一堆，奇奇怪怪的，灾难性的，整了大半天的问题
                        
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